Computational detailsElectronic structure calculations based on density functional theory (DFT) [1] are carried out using accurate plane augmented wave (PAW) method [2] as implemented in Vienna Ab initio Simulations Package (VASP) [3]. The exchange-correlation energy functional as given by Perdew, Burke and Ernzerhof (PBE) [4] is used, since it is known to provide descent estimates for electronic properties. The valence electronic configurations used for Zn and S are 4s 2 3d 10